Stable 3,6-Linked Fluorenyl Radical Oligomers with Intramolecular

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Stable 3,6-Linked Fluorenyl Radical Oligomers with Intramolecular Anti-ferromagnetic Coupling and Polyradical Characters Xuefeng Lu, Sangsu Lee, Jun Oh Kim, Tullimilli Y. Gopalakrishna, Hoa Phan, Tun Seng Herng, Zheng Long Lim, Ze-Bing Zeng, Jun Ding, Dongho Kim, and Jishan Wu J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b08138 • Publication Date (Web): 09 Sep 2016 Downloaded from http://pubs.acs.org on September 9, 2016

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Journal of the American Chemical Society

Stable 3,6-Linked Fluorenyl Radical Oligomers with Intramolecular Anti-ferromagnetic Coupling and Polyradical Characters Xuefeng Lu,† Sangsu Lee,‡ Jun Oh Kim,‡ Tullimilli Y. Gopalakrishna,† Hoa Phan,† Tun Seng Herng,§ Zhenglong Lim,† Zebing Zeng,|| Jun Ding,§ Dongho Kim*,‡ and Jishan Wu*,† †Department

of Chemistry, National University of Singapore, 3 Science Drive 3, 117543, Singapore

‡Spectroscopy

Laboratory for Functional π-Electronic Systems and Department of Chemistry, Yonsei University, Seoul 120-749, Korea

§

Department of Materials Science & Engineering, National University of Singapore, 119260, Singapore

||

College of Chemistry and Chemical Engineering, Hunan University, Changsha, 410082, Peoples Republic of China

Supporting Information Placeholder ABSTRACT: Organic radicals display unique physical structures and could become next generation functional materials. However, design and synthesis of stable neutral radicals with a significant polyradical character has been an enormous challenge for chemists. In this work, we synthesized a series of stable 3,6-linked, kinetically blocked fluorenyl radical oligomers up to hexamer (FR-n, n = 1-6). Their ground-state geometric and electronic structures were systematically studied by various experimental methods including X-ray crystallographic analysis, variable temperature nuclear magnetic resonance, electron spin resonance and superconducting quantum interference device measurements, supported by density functional theory and ab initio calculations. Moderate anti-ferromagnetic coupling between the fluorenyl radicals was observed and moderate-to-large diradical and polyradical characters were calculated from dimer onward. Furthermore, their photophysical properties were estimated by steady-state, transient absorption and two-photon absorption measurements, and their electrochemical properties were investigated by cyclic voltammetry/differential pulse voltammetry and spectro-electrochemical measurements. A clear chain length dependence of their optical, electrochemical and magnetic properties was found for the oligomers with an odd or even number of spin centers, respectively.

INTRODUCTION π-Conjugated polycyclic hydrocarbons with an open-shell singlet ground state have recently attracted immense attention due to their unique optical, electronic, and magnetic properties and potential applications in organic electronics, nonlinear optics, spintronics, and energy storage devices.1 So far, various persistent carbon-centered monoradicals such as polychlorotriphenylmethyls,2 cyclopentadienyls3 and phenalenyls4 have been designed and synthesized. Connection of two carbon radical centers via a πconjugated spacer in an anti-ferromagnetic (AFM) coupling mode could lead to a diradical-like structure in singlet ground state, which is called open-shell singlet diradicaloid.1f, 5 Theoretically, a diradical character index (y0) can be used to account for the bond order of the two frontier π-electrons based on the occupation number of the lowest unoccupied natural orbital (LUNO).6 Nakano et al. have nicely correlated the diradical character to the exci-

tation energies, non-linear optical properties and singlet fission phenomenon in the open-shell singlet diradicaloid systems,5 which were also validated by recent experimental studies on relatively stable diradicaloids such as bisphenalenyls,7 quinoidal oligothiophenes and thienacenes,8 zethrenes,9 indenofluorenes,10 anthenes,11 and extended para-quinodimethanes.12 However, so far there have been very few reports on the stable open-shell polyradicaloids with a low-spin ground state and a significant polyradical character.13 Organic carboncentered radicals with a polyradical character, that is, in which multiple π-electrons are not tightly paired into the bonding molecular orbitals, are supposed to be extremely reactive, which makes it more challenging for chemists to design, synthesize, isolate and characterize stable polyradicaloids. In addition, too strong or too weak AFM coupling between the spins would result in a too small or too large polyradical character, respectively,13b,c and thus a

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moderate spin-spin coupling is more desirable for optimal physical properties. Among various carbon-centered monoradicals, cyclopentadienyl radicals as one of the most stable πconjugated hydrocarbon radicals have been extensively studied and some of them have been isolated in crystalline form.3 The stability of the cyclopentadienyl radical center can be effectively improved by kinetic blocking and spin delocalization. As a dibenzo- cyclopentadienyl derivative, the fluorenyl radical is easy to chemically access and has sufficient stability if the 9-position is appropriately substituted. Our group first demonstrated that a 9anthryl group can effectively stabilize fluorenyl monoradical, and the obtained monoradical and diradicals can be even purified by normal silica gel column chromatography.14 Kubo’s group also reported a series of π-extended fluorenyl monoradicals substituted by 9-anthryl groups, which are stable too.15 In this context, a series of 3,6linked fluorenyl radical oligomers FR-n (n=1-6) in which the 9-positions of all fluorenyl radicals are kinetically blocked by 3,5-di-tert-butylphenyl substituted 9-anthryl groups are designed and synthesized as potential stable polyradicaloids (Figure 1). The 3,6-linkage allows an effi-

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cient AFM coupling between the fluorenyl radicals, that is, close-shell quinoidal form can be drawn for the two neighbouring fluorenyl radicals (see representative canonical forms in Figure 1, only trans- forms are drawn). We demonstrated that the recovery of Clar’s aromatic sextet rings (the hexagons shaded in blue colour in Figure 1) in the diradical canonical form can serve as a major driving force to form diradical, and the more sextets gained, the larger the diradical character.9c Taking the dimer FR-2 as an example, two aromatic sextet rings can be gained from the closed-shell form to diradical form, and thus moderate diradical character can be expected for this molecule. Similar analyses can be applied to the higher order oligomers and there is sufficient driving force (two sextets) from the diradical to tetraradical (in FR-4/FR-5) and even to hexaradical form (in FR-6). Therefore, this series of fluorenyl radical oligomers is expected to be stable and show significant diradical and polyradical character. In this Article, we will report comprehensive studies on their synthesis, ground-state geometric and electronic structures, and chain-length dependent optical, electrochemical and magnetic properties by various experimental techniques as well as theoretical methods.

Figure 1. Chemical structures and representative canonical forms of the 3,6-linked fluorenyl radical oligomers.

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Scheme 1. Synthetic route for FR-n (n=1-6). Reagents and conditions: (a) NBS, FeCl3 (cat.), CHCl3; (b) i) n-BuLi, THF; ii) 3,6dibromofluorenone; iii) H2O; (c) NaH, THF, then MeI; (d) Pd(PPh3)4, K2CO3, THF/H2O, reflux; (e) SnCl2, CH2Cl2; (f) B2(Pin)2, Pd(dppf)Cl2CH2Cl2, KOAc, dioxane.

RESULTS AND DISCUSSION Synthesis. The synthetic approach to six oligoradicals (FR-n, n = 1-6) is depicted in Scheme 1. The synthesis of the key building block 4 starting from 9-(3,5-di-tertbutylphenyl)anthracene (1)14 which can be easily converted into 9-bromo-10-(3,5-di-tertbutylphenyl)anthracene (2) by bromination with NBS in 89% yield. Compound 2 was treated with 0.98 equiv of n-BuLi followed by reaction with 3,6-dibromo-9Hfluoren-9-one to give the alcohol compound 3 in 81% yield. The alcohol group was then protected by methyl with iodomethane to provide the key building block 4 in

95% yield. Then, Suzuki coupling reaction between 4 and 0.9 equiv of 4-tert-butylphenylboronic acid afforded the mono-substituted compound 5 (in 54% yield) and the di-substituted compound 6 (in 22% yield). The monobromo- compound 5 was converted into the corresponding boronic ester 7 in 75% yield via palladium catalysed Miyaura borylation. Suzuki coupling reaction between 5 and 7 gave the fluorenyl dimer precursor 8 in 59% yield. Following a similar strategy, the monoboronic ester 11 and 13 were prepared, which were used for the synthesis of fluorenyl tetramer and hexamer

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Figure 2. X-ray crystallographic structures of FR-1 and FR-2 and selected bond lengths (in Å).

precursors 12 and 16, respectively. The trimer and pentamer precursors (10 and 14) were isolated during the Suzuki coupling reaction between the monoboronic esters (7 and 11) with 1 equiv of 5. In the last step, the obtained precursors (6, 8, 10, 12, 14 and 16) were converted into the corresponding radicals FR-1, FR-2, FR-3, FR-4, FR-5, and FR-6 by reduction reaction with SnCl2 in dichloromethane (DCM) in nearly quantitative yield (monitored by thin layer chromatography and MALDITOF mass spectrometry).16 Importantly, all these fluorenyl radical oligomers are stable compounds and can be easily purified by routine silica gel column chromatography, thus facilitating standard structural and physical characterizations. Their MALDI-TOF mass spectra and high-resolution mass spectra (HRMS) were found to be consistent with the target structures (see Supporting Information (SI)). Variable temperature (VT) 1H NMR spectra also supported their structures (vide infra and more in SI). In addition, the structures of radicals FR-1 and FR-2 were further confirmed by X-ray crystallographic analysis (vide infra). All of the intermediates were well characterized by 1H and 13C NMR and HRMS, and were found to be consistent with the proposed structures (see SI). X-ray Crystallographic Analysis. Single crystals for FR-1 and FR-2 suitable for X-ray crystallographic analysis were grown by slow diffusion of acetonitrile into the toluene solutions under natural ambient conditions, and their structures are shown in Figure 2.17 The dihedral angle between the fluorenyl and anthryl moiety in FR-1 is 80° (mean value), and the almost orthogonal conformation indicates that the 9-anthryl group can be used as a kinetic stabilization group for fluorenyl radicals, and at the same time, can improve the solubility. However, the dihedral angle between the fluorenyl and anthryl moiety in the crystal FR-2 is much smaller (59°), presumably

due to the contribution of a quinoidal structure in the FR-2 diradicaloid (Figure 1b). Theoretically, both cisand trans- configurations could exist for FR-2, but only cis- isomer was crystalized. Due to the crystalline packing effects, the bond lengths of the symmetric units are slightly different in both cases. Bond length analysis reveals that the lengths of the C1-C2 bond (1.438 Å) and the C1-C2’ bond (1.431 Å) of the fluorenyl in the crystal FR-1 are almost the same and have a typical C(sp2)-C(sp2) single bond character, but these two bonds of same position in the crystal of FR-2 show quite different bond lengths, that is, the length of the C7-C9 bond (1.448 Å) is significantly longer than that of the C6-C7 bond (1.408 Å), and the length of the C7’-C9’ bond (1.480 Å) is also significantly longer than that of the C6’-C7’ bond (1.427 Å). In addition, significant bond length alternation was observed for the two central benzenoid rings, and all these indicate an AFM bonding between the two spin centers. On the other hand, the length of the C1-C1’ bond linked two fluorenyl groups is 1.455 Å, which is much longer than that of the typical olefins (1.33-1.34 Å), implying that there are moderate contributions of both the diradical form and quinoidal form to the ground-state electronic structure, which is in accordance with its moderate diradical character (vide infra). Theoretical Calculations. To further understand the ground-state geometric and electronic structures of these fluorenyl oligomers, density functional theory (DFT) calculations at the UCAM-B3LYP/6-31G (d,p)18 level were first carried out. To reduce the computational cost, all the tert-butyl groups were replaced with hydrogen atoms. Due to the contribution of the quinoidal structures in the FR-n (n=2-6), there could be various cis-/transisomers for these oligomers. Our calculations show that the cis- and transisomers have nearly identical energy in FR-2. However, the all-cis and the cis-trans isomers

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have a slightly higher energy (2.5 and 1.6 kcal/mol, respectively) than the all-trans isomer in FR-3 (Figure S1 in SI). In cases of the higher order oligomers FR-4 – FR6, only the all-trans isomers were calculated. Our DFT results reveal that the oligomers with even number of fluorenyl units (FR-2, FR-4 and FR-6) have an openshell singlet ground state whereas the oligomers with odd number of fluorenyl units (FR-1, FR-3 and FR-5) have a doublet ground state. Spin-density maps of these radical oligomers in the ground state show that the fluorenyl radicals nicely interact with each other and the spins are delocalized along the whole π-conjugated frameworks (Figure 3). The diradical character index y0, tetraradical character index y1, and hexaradical character index y2 of FR-n (n=2-6) were calculated based on the occupation number of the LUNO, LUNO+1 and LUNO+2, respectively, according to Yamaguchi’s scheme (Table S1 in SI).19 All the oligoradicals, except monoradical FR-1, show moderate-to-large diradical character values (y0 > 0.5). It is noteworthy that a moderate tetraradical character y1 for the higher oligoradicals FR-4 (y1 = 0.45), FR5 (y1 = 0.43), and FR-6 (y1 = 0.56) was also calculated. Furthermore, the radical FR-6 displays a moderate hexaradical character value (y2 = 0.41). Therefore, it is demonstrated that the higher oligomers have moderate polyradical characters due to the moderate AFM coupling between the fluorenyl radicals.

Figure 3. Calculated (UCAM-B3LYP) spin density distributions of the open-shell singlet (FR-2, FR-4, FR-6) or doublet (FR-1, FR-3, FR-5) in all-trans forms. Blue and green surfaces represent α and β spin density distributions with 0.004 au isosurfaces, respectively.

To better address the polyradical characters and excitation energies of these oligomers, the analysis of the natural orbital occupations was carried out using restricted active space spin flip method (RAS-SF/6-31G*), which can properly deal with the strongly correlated electrons.20, 9e, 13c The radical characters in the ground state were estimated by the number of unpaired electrons (NU).21 Since the calculated occupation numbers of HONO-i and LUNO+i (i = 0-2) also meet the basis of Yamguchi’s scheme: nHONO-i + nLUNO+i = 2 (Figure 4 and Figure S2 in SI), the multiple radical characters (yi, i = 02) were also calculated by using Yamaguchi’s scheme for comparison (Table 1). Table 1. Calculated (RAS-SF/6-31G*) unpaired electron numbers (Nu) and radical characters (yi, i=0-2) of FR-n (n=2-6). Molecule

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5.05

0.80

0.68

0.59

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1.95

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3.23

0.28

0.24

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Figure 4. Calculated (RAS-SF) natural orbitals and occupation numbers of FR-4 (in all-trans form) in the singlet ground state.

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Journal of the American Chemical Society The NO diagrams and occupancy numbers of FR-n (n=2-6) are shown in Figure 4 and Figure S2 in SI. Taking FR-4 as an example, the unpaired electron density is mainly localized at the four fluorenyl units, indicating its large radical character. Significant occupancy numbers were calculated for the LUNO and LUNO+1, and accordingly, large Nu value of 3.32 was obtained. Similar results were found for all other oligomers. It was found that the radical character Nu increases in the two series of oligomers as the chain length increases, i.e., series A with even number (FR-2, FR-4 and FR-6) and series B with odd number (FR-3 and FR-5) of fluorenyl units, respectively (Table 1). Yamaguchi’s polyradical characters (yi, i = 0-2) calculated by this method show a similar trend (Table 1) and are in agreement with the UCAMB3LYP calculations (Table S1 in SI). Again, FR-4 and FR5 show moderate tetraradical character and FR-6 even exhibits moderate hexaradical character. 2.5

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ture, while the peaks began to emerge and sharpen upon cooling (Figure 6). At 213 K, sharp peaks appeared from fluorenyl backbone that can be reasonably assigned to the proposed structure. This phenomenon is commonly observed in most previously reported open-shell singlet diradicaloids9 due to the existence of a significant amount of thermally excited triplet biradical species, which is a little higher in energy than the singlet diradical. Further decrease of temperature to 183 K led to signal broadening due to enhanced aggregation at lower temperature. Similar NMR broadening at room temperature and sharpening as temperature decreases were observed for the higher oligoradicals (Figure S3-S6 in SI), but the broadening became more serious for FR-3 and FR-5 (due to odd number of spins) and for FR-4 and FR6 (due to polyradical character). ESR measurements of all oligoradicals in toluene displayed strong signals at room temperature even at low temperature (−90 °C in toluene) with ge = 2.0026 (Figure S7 in SI), and the signal intensity gradually decreases as temperature decreases, indicating that they all have a low-spin (singlet or doublet) ground state. The spin concentration of FRn (n=2-6) was estimated to be 0.07, 0.83, 0.17, 0.56 and 0.42, respectively, by using the monoradical FR-1 as standard. It is found that the oligomers with odd number of fluorenyl units show larger spin concentration than those with even number of fluorenyl units. From FR-2 to FR-4 and to FR-6, the spin contraction increases as a consequence of the increased diradical and polyradical characters. From FR-1 to FR-3 and to FR-5, however, the spin concentration decreases presumably due to certain intermolecular or intramolecular spin-spin association in solution.

FR-5

Figure 5. Calculated (RAS-SF) excitation energies from the LS ground state and the HS excited states of FR-n (n=2-6).

The excitation energies from the low-spin (LS) ground state to the high-spin (HS) excited state were also calculated (Figure 5 and Table S2 in SI). In accordance with their large radical characters, the calculated vertical energy gaps from the ground state to the lowest excited state are substantially small in these oligomers (< 1 kcal/mol), and they also drop with extension of molecular size in both series A and series B. The excitation energies to higher excited states are also not so large, indicating that the molecules could be thermally promoted to even higher spin states. Magnetic Properties. The ground-state electronic structures of these oligoradicals were further investigated by magnetic measurements such as VT NMR, electron spin resonance (ESR), and superconducting quantum interference device (SQUID) measurements. Monoradical FR-1 did not show any NMR signals for the fluorenyl core at room temperature and even after cooling to 183 K due to its paramagnetic character. The diradicaloid FR-2 showed broad and weak 1H NMR signals for the fluorenyl core (e.g. protons b, c, e) at room tempera-

1

Figure 6. Variable-temperature H NMR spectra of FR-2 in THF-d8 in aromatic region and assignment of aromatic protons. The resonance assignment referred to the structure shown in Figure 2.

SQUID measurements were performed with the powder samples of FR-n (n=1-6) at 2-380 K (Figure 7) and estimated excitation energies from the LS ground state to the first HS excited state are collected in Table 2. For the monomer FR-1, the product of the molar

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Figure 7. χMT-T curves in the SQUID measurements for the solids of FR-n (n=1-6). The solid lines are the fitting curves according to Bleaney-Bowers equation for FR-2/FR-4/FR-6 and the trimer model for FR-3/FR-5; g-factor was taken to be 2.00.

magnetic susceptibility (χM) by the temperature (T, in K) gave a value of 0.36 emu•K•mol-1 from 25 K to 400 K, which is close to the theoretical spin only value (0.375 emu•K•mol-1) for a S = ½ system. The product χM•T drops at temperature below 25 K as the result of weak intermolecular AFM coupling. For the dimer FR-2, the χM•T increases after 150 K and fitting of the data using BleaneyBowers equation22a based on a singlet-triplet model gave a singlet-triplet energy gap (∆ES-T) of −3.52 kcal/mol with a singlet ground state. Similar trend was observed for the tetramer FR-4 and hexamer FR-6, and both have a singlet ground state, with an estimated ∆ES-T value of -2.94 and 2.65 kcal/mol, respectively. For the trimer FR-3 and pentamer FR-5, due to the existence of odd number of spins, a constant χM•T value of around 0.35 emu•K•mol-1 was observed between 25K and 125 K, followed by χM•T increase with increase of temperature due to the thermal population from doublet ground state to the quartet excited state. The energy gaps were estimated as -2.91 and 1.76 kcal/mol, respectively, by fitting the data with the linear trimer model.22b Similar to the monomer, weak intermolecular AFM coupling was observed below 25 K. Therefore, for the oligomers with either even or odd number of fluorenyl units, the measured energy gaps from the LS ground state to HS excited state decrease with the increase of molecular length, in consistence with their calculated radical characters and vertical energy gaps. Optical Properties. The UV-vis-NIR spectra of this series of fluorenyl-based oligoradicals were measured in toluene (Figure 8 and Figure S8 in SI) at a concentration of ca. 1 × 10-5 M, which exhibit different one-photon absorption (OPA) spectral features as the number of spin centers increased, and the corresponding photophysical data are

listed in Table 2. The FR-1 displayed an intense absorption band with vibronic splitting in the region of 300-400 nm, which is characteristic of that of anthracene.23 A long weak absorption tail into the near infrared (NIR) region was observed due to the existence of one unpaired electron and is similar to our previously reported fluorenyl monoradical analogue.14 In contrast, the absorption spectrum of FR-2 showed a relatively intense band in the NIR region with λmax = 720 nm along with a shoulder at 850 nm extending up to 1000 nm, which is quite different from that of monoradical FR-1, indicating that there is significant AFM coupling interaction between the two radicals. The FR-3 also displayed a relatively intense band in the NIR region as observed in FR-2. It is worth noting that FR-3 exhibited a broader spectral response extending up to 1050 nm with a bathochromic shift of 50 nm compared to that of FR-2, further indicating that there is also significant spin coupling interactions between the three radicals. This optical feature was entirely observed in further higher oligomers FR-4, FR-5, and FR-6. Generally, the absorption spectra and extinction coefficients of the radicals are continuously red-shifted and increased, respectively, as the number of radicals increases, all indicating multiple AFM spin interactions in the ground state. The optical energy gaps (Egopt) of FR-n (n=1-6) were roughly estimated to be 1.27, 1.33, 1.24, 1.26, 1.21 and 1.23 eV, respectively, from the lowest energy absorption onset. Therefore, the optical energy gap decreases with the extension of chain length in each individual series containing even number (FR-2, FR-4 and FR-6) or odd number (FR-1, FR-3 and FR-5) of fluorenyl units. A more accurate chain length dependence will be discussed based on electrochemical energy gaps.

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2.0 1.5 1.0 0.5 0.0 400

600

800

Wavelength / nm

Figure 8. One-photon and two-photon absorption spectra of FR-n (n=1-6). The spectra were recorded in toluene. TPA spectra are plotted at λex/2. a

Table 2. Photophysical, electrochemical and magnetic properties of FR-n (n=1-6). Molecule FR-1

FR-2

FR-3

FR-4

FR-5

FR-6

λabs (nm) 394 544 587 768 394 597 745 396 599 755 955 394 594 735 395 600 755 968

εmax -1 -1 (M cm ) 4 1.63×10 4 0.43×10 4 0.34×10 4 0.08×10 4 1.72×10 4 1.24×10 4 1.91×10 4 3.24×10 4 1.83×10 4 2.08×10 4 1.13×10 4 2.74×10 4 1.92×10 4 2.21×10 4 3.56×10 4 2.23×10 4 2.43×10 4 1.42×10

395 598 739

3.56×10 4 2.43×10 4 2.64×10

4

(2)

τ (ps) 0.30

σ max (GM) 300

E1/2 (V) 0.12

ox

E1/2 (V) -1.10

red

EHOMO (eV) -4.85

ELUMO (eV) -3.75

Eg (eV) 1.10

EC

Eg (eV) 1.27

opt

ΔELS-HS (kcal/mol) -

1.30

600

0.17 0.31

-1.13 -1.32

-4.90

-3.73

1.17

1.33

-3.52

1.00

1100

0.05 0.32 0.46

-1.01 -1.20 -1.44

-4.77

-3.87

0.90

1.24

-2.19

0.50

1500

0.13 0.45

-4.83

-3.82

1.01

1.26

-2.94

0.45

1800

0.00 0.16 0.30 0.47

-4.65

-3.88

0.77

1.21

-1.76

0.40

1800

0.02 0.15 0.26 0.38 0.47

-1.09 -1.30 -1.49 -1.01 -1.10 -1.24 -1.40 -1.55 -1.07 -1.18 -1.30 -1.46 -1.55

-4.74

-3.82

0.92

1.23

-2.65

a

λabs: absorption maximum measured in toluene. εmax: molar extinction coefficient at the absorption maximum. τ is singlet ox red excited state lifetime obtained from TA. E1/2 and E1/2 are half-wave potentials of the oxidative and reductive waves, + respectively, with potentials vs Fc/Fc couple. HOMO and LUMO energy levels were calculated according to equations: EHOMO onset onset onset onset = −(4.8 + Eox ) and ELUMO = −(4.8 + Ered ), where Eox and Ered are the onset potentials of the first oxidative and EC opt reductive redox wave, respectively. Eg : electrochemical energy gap derived from ELUMO−EHOMO. Eg : optical energy gap derived from lowest energy absorption onset in the absorption spectra. ΔELS-HS: the energy difference between the LS ground (2) state and the first HS excited state estimated from SQUID measurements. σ max is the maximum TPA cross section at the wavelength of 1500 nm.

8|Page


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As it is known that an open-shell radical character can enhance the two-photon absorption (TPA) crosssection,24 TPA measurements were conducted for oligoradicals FR-n (n=1-6) by using the open aperture Zscan method in the wavelength range from 1200 to 2200 nm where one-photon absorption contribution is negligible (Figure 8, Table 2, and Figure S9 in SI). The monoradical FR-1 displayed significant TPA response in the region of 1200-1600 nm with a maximum TPA cross-section value (σ(2)max) of 300 GM at 1500 nm. The higher order oligoradicals FR-n (n=2-6) exhibited much broader TPA response region extending up to 2200 nm, with σ(2)max = 600, 1100, 1500, 1800 and 1800 GM at 1500 nm, respectively. Therefore, the σ(2)max follows an order: FR-1 < FR-2 < FR-3 < FR4 < FR-5 ≈ FR-6, and this trend may be correlated to the resonance enhancement of the transitions observed in the OPA spectra and the multiple moderate AFM spin coupling interactions in the higher oligomers. However, the TPA cross-section almost reaches saturation at the pentamer. To investigate the excited-state dynamics of these oligoradicals, the femtosecond transient absorption (TA) measurements were carried out in toluene at room temperature (Figure S10 in SI and Table 2). The TA spectra of FR-1 exhibited ground-state bleaching (GSB) signals around 530-590 nm as well as two relatively higher intensity excited-state absorption (ESA) bands in the 450-530 and 620-750 nm regions (Figure S10 in SI). The excitedstate lifetime (τ) was determined to be 300 fs in the decay profile. This ultrafast excited-state lifetime of FR-1 is considered to reflect an acceleration of the non-radiative decay rates arising from the smaller energy gap between the lowest excited state and the open-shell ground state. The non-fluorescent property of radical FR-1 is also well in agreement with its ultrafast excited-state lifetime. The higher oligoradicals FR-n (n=2-6) showed similar TA spectral features with each other. The TA spectra of FR-n (n=2-6) displayed ground-state bleaching (GSB) signals from 530-850 nm as well as one relatively weaker intensity ESA band in the region of 450-530 nm, which is distinctly different from FR-1. These results are well matched with the steady-state absorptions. Furthermore, ultrafast excited state lifetimes with time constants of 1300, 1000, 500, 450 and 400 fs were observed in the decay profiles of FRn (n=2-6), respectively. Thus, the excited-state lifetimes decrease as the chain length increases, which might be a consequence of the increased diradical and polyradical characters. Electrochemical Properties. To investigate the electrochemical properties of these oligomers, cyclic voltammetry and differential pulse voltammetry measurements were carried out in a typical three-electrode electrochemical cell in a solution of tetrabutylammonium hexafluoro-

phosphate (0.1 M) in dry DCM with a scan rate of 50 mV s1 (Figure 9 and Table 2). The monoradical FR-1 exhibited one reversible oxidation wave at E1/2ox = 0.12 V (vs Fc+/Fc) and one reversible reduction wave at E1/2red = −1.10 V (vs Fc+/Fc), which can be correlated to the cation and anion formation. FR-2 showed two reversible oxidation waves with E1/2ox = 0.17 and 0.31 V, and two reversible reduction waves with E1/2red = -1.13 and -1.32 V, indicating that it can be reversibly oxidized to its radical cation and dication, and reduced into radical anion and dianion. Analogously, FR-3 displayed three reversible oxidation waves with E1/2ox = 0.05, 0.32 and 0.46 V, and three quasi-reversible reduction waves with E1/2red = -1.01, -1.20, and -1.44 V, implying that it can be oxidized to its diradical cation, radical dication, and trication, and reduced into diradical anion, radical dianion and trianion forms. The higher oligoradicals FR-4–FR-6 demonstrated more reductive and oxidative waves due to the diminished Coulomb charge repulsion in the more extended systems. FR-4 could be reversibly oxidized into diradical dication and tetracation via twoelectron transfer at E1/2ox = 0.13 and 0.45 V, and reduced into diradical dianion, radical trianion and tetraanion at E1/2red = -1.09, -1.30 and -1.49 V. For pentamer FR-5, four major oxidation processes and five reduction processes were detected, and for hexamer FR-6 five major quasireversible oxidation processes and five reduction processes were detected. The HOMO and LUMO energy levels of these oligomers were determined from the onset potentials of the first oxidation and reduction waves, and the electrochemical energy gaps (EgEC = LUMO-HOMO) thus can be determined accordingly, which are in good accordance with the optical energy gaps (Table 2). It is worth noting that good chain-length dependences of the HOMO/LUMO energy levels and EgEC were found separately for the oligomers with either even (series A) or odd number (series B) of fluorenyl units. In each series, with the extension of the chain length, the HOMO energy level increases and the LUMO energy level decreases, leading to a convergence of the EgEC value. Two linear EgEC ∼ 1/n plots (n is the number of the fluorenyl unts) were found for these two series (Figure 10), which is similar to many other πconjugated oligomers.25 The oligomers with even and odd number of spins are quite different from each other due to their different electronic structures. Linear extrapolation of the two plots to n= ∞ implies that the corresponding infinite polymers with even and odd number of fluorenyl radicals may have an electrochemical energy gap of 0.84 eV and 0.59 eV, respectively. This, however, may not be true as like other conjugated polymers, a saturation of the energy gap could be observed at certain effective conjugation length.

9|Page



FR-1-CV FR-1-DPV

Current

Current

Current

FR-2-CV FR-2-DPV

FR-3-CV FR-3-DPV

-1.5

-1.0

-0.5

0.0

0.5

+

-1.5

-1.0

-0.5

0.0

+

0.5

-0.5

0.0

-0.5

0.5

0.0

+

0.5

Current

Current

Current

-1.0

-1.0

Potential (V) vs. Fc /Fc

FR-5-CV FR-5-DPV

FR-4-CV FR-4-DPV -1.5

-1.5



-1.5

+

-1.0

-0.5

0.0

FR-6-CV FR-6-DPV 0.5

+



-1.5

-1.0

-0.5

0.0

0.5

+


Figure 9. Cyclic voltammograms (black curves) and differential pulse voltammograms (red curves) of FR-n (n=1-6) in DCM solutions. 1.2

odd even

FR-2

1.1

FR-4

FR-1

EC

(eV)

1.0

FR-6 0.9

Eg

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 13

FR-3

0.84 eV 0.8

FR-5

0.7 0.6

0.59 eV 1/6

1/5

1/4

1/3

1/2

1/1

1/n

Figure 10. Chain-length (n) dependence of the electrochemiEC cal energy gaps (Eg ) for FR-n (n=1-6). Extrapolation of the linear plot to n = ∞ gives the estimated energy gaps for the infinite polymers with odd (black line) or even (red line) spins.

The multiple-step amphoteric redox behaviour of these oligomers allows us to probe their charged states either by electrochemical or chemical oxidation/reduction. Spectro-electrochemical measurements were first conducted in dry DCM solutions and the absorption spectra of the major oxidized species at different electrode potentials (vs Fc+/Fc) are shown in Figure 11. FR-1 can be easily oxidized into cation, with a broad and intense absorption with λmax at 1075 nm. FR-2 is oxidized to the radical cation at 0.3 V, with two intense absorption bands in the NIR region (λmax

= 1245, 2200 nm), and then further oxidized into dication at 0.8 V (λmax = 1166 nm). FR-3 can be oxidized into diradical cation at 0.2 V (λmax = 2128 nm), radical dication at 0.4 V (λmax = 1243, 2152 nm) and then to trication at 0.9 V (λmax = 1160 nm). In accordance with the CV data, FR-4 undergoes two-electron oxidation to diradical dication at 0.4 V (λmax = 1110, 1670, 2340 nm), and then two-electron oxidation to the tretracation at 0.9 V (λmax = 1195 nm). FR-5 can be oxidized to tetraradical cation at 0.3 V (λmax = 1204, 1790 nm), and then multiple electron transfer to the final pentacation at 0.9 V (λmax = 1192 nm). Similarly, FR-6 is oxidized into pentaradical cation at 0.4 V (λmax = 1030, 1680 nm) and then to hexacation at 1.0 V (λmax = 1195 nm). In general speaking, all the full oxidized cations show similar band structure and absorption wavelength, and intermediate radical cations exhibit very long wavelength absorption extending to 3000 nm, indicating their mixedvalence character. It turned out that electrochemical reduction did not have a good control on the reduction process, therefore, chemical reduction of these oligomers were conducted in dry DCM and followed by UV-vis-NIR spectroscopic measurements. Using strong reductant such as KO2 together with 18-crown-8 gave fully reduced anions, and all of them show very similar band structure with absorption maximum at 384 nm, correlated to the absorption of an aromatic fluorenyl anion (Figure S11 in SI). The reductions of FR-n (n=2-6) by CoCp2 in DCM stopped at the (multiple)radical monoanions stage, which all show a broad NIR absorption band with λmax = 1947, 1979, 2340, 1866 and 1869 nm, respectively (Figure S12 in SI).

10 | P a g e


FR-2 Radical Cation (0.3 V) Dication (0.8 V)

2128 2152

1243 1160

750 582

2220

1245 1166

745

605

Abs.

1075

612

FR-3 Diradical Cation (0.2 V) Radical Dication (0.4 V) Trication (0.9 V)

545 587

530

Abs.

394

FR-1 Cation (0.8 V)

1600

500

1000

1500 2000 Wavelength (nm)

500

1000

500

1000


3000

500

1000


2500

2500

3000

FR-6 Tetraradical Dication (0.4 V) Hexacation (1.0 V)

3000

500

1000

1680

1195 1030

1192

571

Abs.

1204

2500

3000

FR-5 Tetraradical Cation (0.3 V) Pentacation (0.9 V) 736


2340

1670

1110

1195

730

FR-4 Diradical Dication (0.4 V) Tetracation (0.9 V)

2500

738

1400

568

800 1000 1200 Wavelength (nm)

Abs.

600

1790

400

Abs.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Abs.

Page 11 of 13


2500

3000

Figure 11. UV-vis-NIR spectra of neutral and oxidized species of FR-n (n=1-6) during the spectro-electrochemical measure+ ments. The applied potential is vs E(Fc /Fc).

CONCLUSIONS In conclusion, a series of 3,6-linked fluorenyl radical oligomers were synthesized by step-wise reactions. They are exceptionally stable mainly due to kinetic blocking by the bulky anthryl groups. Moderate intramolecular AFM coupling between the radical centers was observed, which leads to moderate polyradical characters, low excitation energies, long wavelength one-photon absorption, large TPA cross sections and multiple-stage amphoteric redox behaviour. Clear chain-length dependence of physical properties can be seen for this unique polyradical-like system. Our research provides a general design strategy for open-shell low-spin systems with polyradical characters, which open the opportunities to access stable organic materials with unique optical, electronic and magnetic properties. ASSOCIATED CONTENT Supporting Information. Synthetic procedures and characterization data of all new compounds; details for all physical characterizations and theoretical calculations; and additional spectroscopic and X-ray crystallographic data. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]; [email protected] ACKNOWLEDGMENT

J.W. acknowledges financial support from the MOE Tier 3 programme (MOE2014-T3-1-004) and MOE Tier 2 grant (MOE2014-T2-1-080). The work at Yonsei University was supported by the Global Frontier R&D Program on Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Science, ICT & Future, Korea (NRF-2014M3A6A7060583). Z.Z. acknowledges the financial support from the Startup Fund (531109020043) from Hunan University. We thank Dr Bruno Donnadieu for X-ray crystallographic analysis. REFERENCES (1) (a) Sun, Z.; Ye, Q.; Chi, C.; Wu, J. Chem. Soc. Rev. 2012, 41, 7857. (b) Shimizu, A.; Hirao, Y.; Kubo, T.; Nakano, M.; Botek, E.; Champagne, B. AIP Conf. Proc. 2012, 1504, 399. (c) Abe, M. Chem. Rev. 2013, 113, 7011. (d) Sun, Z.; Zeng, Z.; Wu, J. Acc. Chem. Res. 2014, 47, 2582. (e) Kubo, T. Chem. Rec. 2015, 15, 218. (f) Zeng, Z.; Shi, X.; Chi, C.; López Navarrete, J. T.; Casado, J.; Wu, J. Chem. Soc. Rev. 2015, 44, 6578. (g) Kubo, T. Chem. Lett. 2015, 44, 111. (2) (a) Ballester, M. Acc. Chem. Res. 1985, 18, 380. (b) Veciana, J.; Ratera, I. Polychlorotriphenylmethyl radicals: towards multifunctional molecular materials, chapter 2 in the book Stable Radicals; Hicks, R. G. Eds.; John Wiley & Sons, Ltd (Wiltshire): 2010, p 33. (3) (a) Sitzmann, H.; Boese, R. Angew. Chem. In. Ed. Engl. 1991, 30, 971. (b) Sitzmann, H.; Bock, H.; Boese, R.; Dezember, T.; Havlas, Z.; Kaim, W.; Moscherosch, M.; Zanathy, L. J. Am. Chem. Soc. 1993, 115, 12003. (c) Jux, N.; Holczer, K; Rubin, Y. Angew. Chem. In. Ed. Engl. 1996, 35, 1986. (4) Morita, Y.; Nishida, S. Phenalenyls, cyclopentadienyls, and other carbon-centered radicals, chapter 3 in the book Stable Radicals; Hicks, R. G. Eds.; John Wiley & Sons, Ltd (Wiltshire): 2010, p 81. (5) Nakano, M. Excitation Energies and Properties of Open-Shell Singlet Molecules; Springer: New York, 2014.

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Stable 3,6-Linked Fluorenyl Radical Oligomers with Intramolecular

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